The present invention relates to an optical recording apparatus for forming a row of pits in an optical recording medium in accordance with signals in the form of a pulse train, and relates to an optical disk produced by this optical recording apparatus.
Optical disks and magnetooptical disks respectively utilizing optical and magnetooptical signal recording/reproducing methods are known in the art. Such optical recording media include those of the so-called ROM type, P-ROM type, RAM type, etc. In order to realize interchangeability among the various optical recording media, a sampling servo system disclosed, for example, in Japanese Patent Unexamined Publication No. 63-53760, has been proposed. In the sampling servo system, a servo region including so-called pre-pits for forming servo signals (hereinafter, referred to as a pre-format signal) at predetermined intervals or predetermined angles is previously recorded on a concentric or spiral track on a disk, similarly to the case of a so-called sector servo system used in a hard disk or a magnetic disk, so that when the disk is rotated servo control is continuously performed by sampling and holding the foregoing discrete servo signals.
The sampling servo system uses a 4/15 modulation system which has a feature in that logic "1" can be continued, at maximum, in three of the data bits carried by a recording signal. An example of such a recording signal is shown in FIG. 7 of Japanese Patent Application No. 63-278200 (corresponding to U.S. patent application Ser. No. 344,866). Therefore, the pre-format signal includes a 1T signal having a basic pulse duration corresponding to one bit of logic "1", a 2T signal having a pulse duration twice as large as the basic pulse duration corresponding to two bits of logic "1", and a 3T signal having a pulse duration three times as large as the basic pulse duration corresponding to three bits of logic "1".
In a process for producing a master disk having pre-format signals formed thereon, an optical modulator is controlled on the basis of a pre-format signal having a predetermined level so as to modulate the intensity of a laser beam. The modulated laser beam is focused onto the disk to thereby form pits on the disk. The various length pits formed in the disk surface also differ in width and in length to width ratio, as shown in FIG. 3. The width and length among the 1T, 2T and 3T pits satisfy the relations W.sub.1 &lt;W.sub.2 &lt;W.sub.3 and L.sub.1 &lt;L.sub.2 &lt;L.sub.3, respectively, where W.sub.1, W.sub.2 and W.sub.3 represent the respective widths in the radial direction of pits 1T, 2T and 3T, and L.sub.1, 2L.sub.2 and 3L.sub.3 represent the respective lengths in the circumferential directions of pits 1T, 2T, and 3T.
The respective widths of the 2T and 3T pits are unevenly increased, so that a signal level phenomenon called "hump" appears in a pit readout signal as shown in FIG. 4. The hump becomes one of the causes of a data read error. If the cutting power of the laser beam is set sufficiently low to suppress any increase in pit width, the level of a crosstalk component contained in a read signal obtained from a 1T clock pit is reduced. The crosstalk in off-track is used for extracting a system clock when the tracking servo is released to perform high-speed access as shown in FIG. 5, and therefore the reduction in level of the crosstalk component is disadvantageous. Further, since a 1T signal is standardized so as to have a half-amplitude level, it is difficult to eliminate the hump phenomenon.